1. Field of the Invention
The invention relates to a display device comprising a display tube for displaying video information, at least a line deflection coil and at least a field deflection coil, a deflection unit having a first output for applying a line deflection signal to the line deflection coil and a second output for applying a field deflection signal to the field deflection coil, a memory having an input for receiving and storing the video information under the control of a first clock signal and an output for reading the video information to be applied to the display tube under the control of a modulatable second clock signal, and a correction circuit for correcting a position error upon display of the video information on the display tube, with an input for receiving an input signal related to the line deflection current, said correction circuit comprising a modulatable clock generator for generating the second clock signal to be applied to the memory and an output for applying said second clock signal to the memory.
The invention also relates to a correction circuit for use in a display device.
2. Description of the Related Art
A display device and a correction circuit of this type are known from U.S. Pat. No. 4,673,986. In this known display device, a geometry model is used for the geometry correction, receiving a signal related to the line deflection current at a first input and a field deflection signal generated by the deflection unit at a second input. The geometry model determines the geometry-corrected deflection signals with reference to these input signals. In this known display device, a control signal for the modulatable clock generator is determined with reference to the line deflection signal generated by the deflection unit and a signal fed back via an output of the geometry model.
In the definition of a video signal (as transmitted), it is assumed that the picture to be displayed is scanned in an orthogonal pattern at a constant rate of the write spot (=the position where the electron beam impinges upon the phosphor layer). Theoretically, the line and field deflection fields are sawtooth-shaped, at 15625 Hz and 50 Hz, respectively. However, a rectangular picture will not be produced due to the shape of the display tube (too flat). The picture will be pincushion-distorted and the horizontal and vertical components of the spot rate will not be constant either. Other causes of the geometry errors are the position of the electron gun/guns (too close to the display screen, or the display screen is "too flat"), the shape of the deflection fields of the line and field deflection coils, the variation of the deflection sensitivity due to a change of the EHT which results in a phenomenon referred to as breathing, and the non-ideal waveforms of the line and field deflection currents. The picture is then displayed in a geometrically distorted form.
The consistency of the spot rate is (always) improved by giving the currents a sinusoidal shape instead of a sawtooth shape, which is referred to as the S correction. However, since the horizontal deflection current has the shape of an attenuated sine, an (asymmetrical) linearity remains to be corrected. Thus, there will always be errors of a higher order which are still to be corrected.
Moreover, the geometry is disturbed because the line frequency of the incoming video signal is varied.
It is not easy to correct geometry errors by adapting the horizontal deflection current. In fact, very high voltages and very large currents occur in the line deflection circuit which produces much energy. Consequently, the current is difficult to influence and the components used are large, hot, expensive and vulnerable.
Another way of geometry correction is to refrain from correcting the deflection current (or currents) and to adapt the video signal instead, so that the correct information is written at the correct position.
Horizontal correction is most necessary and also easiest, because the information is then only to be shifted within one line. This is referred to as clock modulation. Alternatively, a new pixel content could be computed on the basis of the contents of a plurality of adjacent pixels, which is referred to as scan rate conversion.
In the circuit described in the above-mentioned Patent, a clock modulator is used in which the line deflection signal is applied in an unchanged (uncorrected) form to the line deflection coil. A signal related to the line deflection is applied to the correction circuit. With reference to this signal, with reference to the signal at the output of the (line) deflection unit and with reference to a hardware geometry model, the correction circuit determines a position error signal. This position error signal is applied as an input signal to the clock generator which determines the rate at which the video signal is read from the memory.
A drawback of this known correction circuit is that the geometry correction is carried out by means of an analog mathematical circuit generating two functions of the input signals. This is only possible in display tubes having very simple geometry errors so that this is not a realistic solution.
A further drawback is that the correction circuit in the known display device is inaccurate and expensive, and that the performance of the correction circuit is too much dependent on the elements of the correction circuit.